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[email protected] tabbypurr@gmail.com is offline
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Default how hot do you run you CH boiler

On Sunday, 13 January 2019 10:34:57 UTC, Harry Bloomfield wrote:
tabbypurr wrote :


If the Vaillant has 2 separate call for heat signals for DHW & CH it would be
simple enough to split the temp, at least if the boiler has 2 separate temp
settings for the 2 circuits, and the Vaillant 400 series I'm familiar with
does. Where is the problem happening?


It is a Vaillant 418 ECOfit Pure. It only has one input for call for
heat, a permanent supply, plus an output for the pump. It also only has
a single temperature setting, which I presently have set for max (75C).
From cold, the boiler runs flat out until its output nears 75C, then it
begins to modulate its heat output gradually down.

We like the HW hot, so have it set at 75C for HW. The HW is set to come
on for an hour per day, to bring the tank up to temperature, though
when more HW is needed like when taking a bath, we press a 1 hour
override button.

75C is unnecessary for the CH and not as economic as 60C would be, so I
am looking for a workaround to allow for split HW v CH boiler
temperatures.

My first attempt at a split was a disaster...

I added an adjustable thermostat to the CH pipe, just past the 3 port
valve, wired to interrupt the CH call for heat wire when the pipe got
to 60C. Which it did perfectly, but that caused the boiler to cycle
frequently on and off, as the pipe temperature rapidly cooled and
reheated. It also defeated the boiler's ramping down (modulating) its
output.

Another way I had thought of, was to fool the boilers own output
temperature sensor, so when the call is for CH, it sees a false higher
than actual temperature. I could probably manage that, with an extra
call for CH wire back to the boiler (easy), a simple relay and a bit of
jiggery pockery in the boiler sensor circuit. As it is a brand new
boiler, I am reluctant to start modifying it and risk the guarantee.


So if you'd bought a used 418 ecotec you'd have just what you need.
There is one option that may work in some cases: reduce the maximum power output the boiler is allowed to deliver. Then you get efficiency from modulation & cycling with pump overrun becomes acceptable.


NT